Typically, in a distal end portion of an endoscope, structural objects such as an illumination optical system for illuminating a subject, an observation optical system including, at a distal end face thereof, an optical member for receiving incoming light from the subject, a channel that guides a treatment instrument, and a nozzle for cleaning dirt adhering to a lens surface of the image pickup optical system are provided.
An endoscope including an observation optical system that allows magnified close observation allows not only normal observation but also close observation by moving a part of lenses in the observation optical system and thereby changing a focal length. However, since a same illumination optical system is used for both normal observation and close observation, in particular, in a close observation in which a distance to a subject is no more than 2 mm, a peripheral part of a screen is brighter than a center of the same because of the layout of the illumination optical system. Therefore, when a subject to be focused on is positioned at the center part, a favorable observation cannot be performed because of insufficient distribution of illuminating light.
PTL 1 discloses an endoscope that provides only small illumination unevenness in a magnified close observation and thus enables illumination of a subject with sufficient brightness by setting a variable range of a best focal position within a range in which illuminating light beams emitted from a plurality of illumination windows overlap.
Also, PTL 2 discloses an endoscope that minimizes unevenness in an observation area in close observation by providing illumination means at respective positions on opposite sides of an observation optical system at which an illuminance of a peripheral part of the observation area is no more than twice an illuminance of a center part of the observation area.